Xiewen Wen

Ion clustering in aqueous solutions probed with vibrational energy transfer

Despite prolonged scientific efforts to unravel the hydration structures of ions in water, many open questions remain, in particular concerning the existences and structures of ion clusters in 1∶1 strong electrolyte aqueous solutions. A combined ultrafast 2D IR and pump/probe study through vibrational energy transfers directly observes ion clustering in aqueous solutions of LiSCN, NaSCN, KSCN and CsSCN. In a near saturated KSCN aqueous solution (water/KSCN molar ratio = 2.4/1), 95% of the anions form ion clusters. Diluting the solution results in fewer, smaller, and tighter clusters. Cations have significant effects on cluster formation. A small cation results in smaller and fewer clusters. The vibrational energy transfer method holds promise for studying a wide variety of other fast short-range molecular interactions.

Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures

Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Mode-specific intermolecular vibrational energy transfer. I. Phenyl selenocyanate and deuterated chloroform mixture

Vibrational energy transfer from the first excited state (2252 cm−1) of the C–D stretch of deuterated chloroform (DCCl3) to the 0-1 transition (2155 cm−1) of the CN stretch of phenyl selenocyanate (C6H5SeCN) in their 1:1 liquid mixture was observed with a pump/probe two-color two dimensional infrared spectroscopic technique. The mode-specific energy transfer can occur mainly because of the long vibrational lifetime of the CN stretch first excited state (∼300 ps) and the relatively strong hydrogen-bond between the C–D and CN (calculated H-bond formation energy in gas phase ∼−5.4 kcal/mol). The mode-specific energy transfer is relatively low efficient (only ∼2%), which is mainly because of the relatively short vibrational lifetime (∼9 ps) of the C–D stretch first excited state and the big donor/acceptor energy mismatch (97 cm−1) and the slow transfer kinetics (1/kCD→CN=330 ps).

Mode-specific intermolecular vibrational energy transfer. II. Deuterated water and potassium selenocyanate mixture

Vibrational energy transfer from the first excited state (2635 cm(-1)) of the O-D stretch of deuterated water (D(2)O) to the 0-1 transition (2075 cm(-1)) of the CN stretch of potassium selenocyanate (KSeCN) in their 2.5:1 liquid mixture was observed with a multiple-mode two dimensional infrared spectroscopic technique. Despite the big energy mismatch (560 cm(-1)) between the two modes, the transfer is still very efficient with a time constant of 20 ps. The efficient energy transfer is probably because of the large excitation coupling between the two modes. The coupling is experimentally determined to be 176 cm(-1). An approximate analytical equation derived from the Landau-Teller formula is applied to calculate the energy transfer rate with all parameters experimentally determined. The calculation results are qualitatively consistent with the experimental data.

Nonresonant and Resonant Mode-Specific Intermolecular Vibrational Energy Transfers in Electrolyte Aqueous Solutions

The donor/acceptor energy mismatch and vibrational coupling strength dependences of interionic vibrational energy transfer kinetics in electrolyte aqueous solutions were investigated with ultrafast multiple-dimensional vibrational spectroscopy. An analytical equation derived from the Fermis Golden rule that correlates molecular structural parameters and vibrational energy transfer kinetics was found to be able to describe the intermolecular mode specific vibrational energy transfer. Under the assumption of the dipole-dipole approximation, the distance between anions in the aqueous solutions was obtained from the vibrational energy transfer measurements, confirmed with measurements on the corresponding crystalline samples. The result demonstrates that the mode-specific vibrational energy transfer method holds promise as an angstrom molecular ruler.

Mapping Molecular Conformations with Multiple-Mode Two-Dimensional Infrared Spectroscopy

The multiple-mode two-dimensional infrared (2D-IR) spectrum in a broad frequency range from 1000 to 3200 cm(-1) of a 1-cyanovinyl acetate solution in CCl(4) is reported. By analyzing its relative orientations of the transition dipole moments of normal modes that cover vibrations of all chemical bonds, the three-dimensional molecular conformations and their population distributions of 1-cyanovinyl acetate are obtained, with the aid of quantum chemistry calculations that translate the experimental transition dipole moment cross angles into the cross angles among chemical bonds.

Cation Effects on Rotational Dynamics of Anions and Water Molecules in Alkali (Li+, Na+, K+, Cs+) Thiocyanate (SCN-) Aqueous Solutions

Waiting time dependent rotational anisotropies of SCN(-) anions and water molecules in alkali thiocyanate (XSCN, X = Li, Na, K, Cs) aqueous solutions at various concentrations were measured with ultrafast infrared spectroscopy. It was found that cations can significantly affect the reorientational motions of both water molecules and SCN(-) anions. The dynamics are slower in a solution with a smaller cation. The reorientational time constants follow the order of Li(+) > Na(+) > K(+) ~/= Cs(+). The changes of rotational time constants of SCN(-) at various concentrations scale almost linearly with the changes of solution viscosity, but those of water molecules do not. In addition, the concentration-dependent amplitudes of dynamical changes are much more significant in the Li(+) and Na(+) solutions than those in the K(+) and Cs(+) solutions. Further investigations on the systems with the ultrafast vibrational energy exchange method and molecular dynamics simulations provide an explanation for the observations: the observed rotational dynamics are the balanced results of ion clustering and cation/anion/water direct interactions. In all the solutions at high concentrations (>5 M), substantial amounts of ions form clusters. The structural inhomogeneity in the solutions leads to distinct rotational dynamics of water and anions. The strong interactions of Li(+) and Na(+) because of their relatively large charge densities with water molecules and SCN(-) anions, in addition to the likely geometric confinements because of ion clustering, substantially slow down the rotations of SCN(-) anions and water molecules inside the ion clusters. The interactions of K(+) and Cs(+) with water or SCN(-) are much weaker. The rotations of water molecules inside ion clusters of K(+) and Cs(+) solutions are not significantly different from those of other water species so that the experimentally observed rotational relaxation dynamics are only slightly affected by the ion concentrations.

Ultrafast multiple-mode multiple-dimensional vibrational spectroscopy

Ultrafast multiple-dimensional vibrational spectroscopy has been extensively applied to studies of molecular structures and dynamics in condensed phases. Along with the developments of new laser sources and new concepts, increasing improvements and applications of this technique have brought the understanding of molecular systems to a new level. In this review, we first briefly introduce the basic concepts, experimental setups and applications of the technique. The most recent progresses in applying vibrational energy transfers to determine intermolecular distances and vibrational couplings to determine three dimensional molecular conformations with our high power multiple-mode multiple-dimensional vibrational spectroscopy are then introduced in more details.

Probing Ion/Molecule Interactions in Aqueous Solutions with Vibrational Energy Transfer

Interactions between model molecules representing building blocks of proteins and the thiocyanate anion, a strong protein denaturant agent, were investigated in aqueous solutions with intermolecular vibrational energy exchange methods. It was found that thiocyanate anions are able to bind to the charged ammonium groups of amino acids in aqueous solutions. The interactions between thiocyanate anions and the amide groups were also observed. The binding affinity between the thiocyanate anion and the charged amino acid residues is about 20 times larger than that between water molecules and the amino acids and about 5-10 times larger than that between the thiocyanate anion and the neutral backbone amide groups. The series of experiments also demonstrates that the chemical nature, rather than the macroscopic dielectric constant, of the ions and molecules plays a critical role in ion/molecule interactions in aqueous solutions.

Molecular Distances Determined with Resonant Vibrational Energy Transfers

In general, intermolecular distances in condensed phases at the angstrom scale are difficult to measure. We were able to do so by using the vibrational energy transfer method, an ultrafast vibrational analogue of Förster resonance energy transfer. The distances among SCN(-) anions in KSCN crystals and ion clusters of KSCN aqueous solutions were determined with the method. In the crystalline samples, the closest anion distance was determined to be 3.9 ± 0.3 Å, consistent with the XRD result. In the 1.8 and 1 M KSCN aqueous solutions, the anion distances in the ion clusters were determined to be 4.4 ± 0.4 Å. The clustered anion distances in aqueous solutions are very similar to the closest anion distance in the KSCN crystal but significantly shorter than the average anion distance (0.94-1.17 nm) in the aqueous solutions if ion clustering did not occur. The result suggests that ions in the strong electrolyte aqueous solutions can form clusters inside of which they have direct contact with each other.